The radioactive tracer used during nuclear heart scanning exposes the body to a very small amount of radiation. No long-term effects have been reported from these doses.
Radiation dose might be a concern for people who need multiple scans. However, advances in hardware and software may greatly reduce the radiation dose people receive.
Some people are allergic to the radioactive tracer, but this is rare.
If you're pregnant, tell your doctor or technician before the scan. It might be postponed until after the pregnancy.